The present invention relates to an extrusion system containing a fixed center die module which requires no mechanical adjustments, for continuously extruding a molten material or a formable mass to provide a tubular structure having a predetermined cross-section size and a uniform wall gauge concentricity. More particularly, the present invention relates to a crosshead extrusion system.
Hoses made primarily of polymeric materials are widely used for transporting various fluids such as water, gasoline, coolants, power steering fluids, transmission fluids, etc. Typically, these hoses are used in automobiles, trucks, buses, aircraft, locomotives and water craft to transmit such fluids. Generally, for such applications it is necessary to provide high pressure hoses having multiple layers, including reinforcing layers, in order for the hoses to withstand the pressures and temperatures involved, and to meet the various federal and state emission regulations, and still exhibit dimensional stability and rugged durability required for today""s fuel powered internal combustion engines. For example, U.S. Pat. No. 4,330,017 to Satoh describes an automobile fuel hose comprising a two-ply rubber tube covered with a reinforcing fiber layer and further with a protective rubber layer. The hose is constructed from a very thin inner layer of a fluorine-containing rubber which is resistant to fuel, and an outer layer of a synthetic rubber such as epichlorine rubber or ethylene-acrylic rubber which is less resistant to gasoline, but is superior in cold resistance. The extrusion system of the present invention can be used in the manufacture of any polymeric hose, e.g. garden hoses, air pressure hoses, and industrial hoses such as those used in connection with hydraulic presses and the like.
U.S. Pat. No. 5,566,720 to Cheney et al. proposes a multiple layer hose comprising a first layer made up of a melt processible fluoroplastic terpolymer wherein the innermost surface is capable of prolonged exposure to hydrocarbon-containing fluids, and a second layer composed of a resinous thermoplastic material such as polyamide which is bonded directly to the outwardly oriented surface of the first layer.
U.S. Pat. No. 5,524,673 to Noone et al. describes an elongated tubing which includes a first layer of an extrudable, melt-processible thermoplastic material, and a second layer of an extrudable, melt-processible thermoplastic material wherein the second layer is capable of sufficiently permanent laminar adhesion with the first layer to prevent delamination during the desired lifetime of the tubing. At least one of the first and second layers is resistant to permeation of hydrocarbons. In addition to the first and second layers, the tubing includes a third layer adhered to the second layer.
Typically, multi-layer tubular structures are manufactured using extrusion processes and, more particularly, using crosshead extrusion processes as described in U.S. Pat. No. 4,361,455 to Arterburn. Such crosshead extrusion systems are employed not only in the manufacture of rubber tubes and hoses, but also for coating cylindrical work pieces with highly viscous materials such as unvulcanized rubber as described in U.S. Pat. No. 4,832,588 to Rasmussen, and in the manufacture of coated wire and cables as described in U.S. Pat. Nos. 5,183,669; 5,780,066; 5,882,694; and 5,980,226 all to Guillemette; and 5,830,516 to McAlpine et al.
Typically, in the manufacture of rubber tubing or rubber-coated wire, the molten rubber material is extruded by means of a crosshead extrusion system which receives a stream of molten rubber material and causes the molten rubber material to be distributed around the circumference of a wire or tube. In the present invention, the term molten is used to define a formable mass and is not intended to specifically suggest that the formable rubber material is in a molten or liquid state. A variety of crosshead devices have heretofore been known in the art for manufacturing hoses and for applying rubber coatings around the circumference of a wire or tubular member. Furthermore, it is known in the art to simultaneously extrude more than one layer of molten rubber to provide a multi-layer tube. Such a process is accomplished by means of multi-layer crosshead extruder devices such as that described in U.S. Pat. No. 4,798,526 to Briggs et al. Such devices typically utilize two or more crosshead extruders for co-extrusion of multiple layers.
It is also known in the art that it is extremely difficult to establish an even and balanced flow of molten material around the circumference of a wire, mandrel or tube. For example, it is known that conventional crosshead extrusion methods pose several inherent problems such as lengthy xe2x80x9cset-upxe2x80x9d time which is the time spent adjusting the extruder for a desired circumferential size and uniform wall gauge concentricity. After the adjustments are made and remade several times and the xe2x80x9cset-upxe2x80x9d is finally complete, the material being extruded from xe2x80x9cset-upxe2x80x9d mode to xe2x80x9cfull runxe2x80x9d mode has, typically, become hotter on the inside, or extrude side, of the crosshead, thus causing concentricity of the tubular structure to be compromised. In addition, the overheated plastic or rubber material frequently becomes discolored and exhibits other undesirable characteristics. Furthermore, it is known that splitting and reblending of the molten material can cause the molten material to blend together unevenly forming undesirable weld or joint lines in the finished product. Accordingly, there is a need for an extrusion system which overcomes the inadequacies and undesirable characteristics of the prior art extrusion systems.
The present invention provides a new and improved crosshead extrusion system wherein the crosshead extruder housing is designed to accept a fixed center or non-adjustable die module which is capable of balancing the flow of molten material to the die and maintaining this balance from a xe2x80x9cset-upxe2x80x9d mode to a xe2x80x9cfull-runxe2x80x9d mode. Die modules are known, however, such die modules are notorious for requiring continuous adjustment of the die module in the housing in order to provide hoses having acceptable cross-section size with a uniform wall gauge concentricity. It has now been found that a tubular member having a predetermined cross-section size and a uniform wall gauge concentricity can be extruded in an extrusion system, particularly, a crosshead extrusion system which employs the die module or torpedo of the present invention. The die module of the present invention has a fixed center and requires no mechanical adjustments while providing a balanced flow of molten or formable material to the die and maintains this balance from a xe2x80x9cset-upxe2x80x9d mode to a xe2x80x9cfull-runxe2x80x9d mode. The tubular member formed using the extrusion system of the present invention has a predetermined cross-section size and a uniform wall gauge concentricity.
A critical feature of the new crosshead extrusion system is the construction of a fixed center die module or xe2x80x9ctorpedoxe2x80x9d which resides inside the extruder housing. The torpedo is designed to split the molten material coming from the screw into two substantially equal parts or controlled flow, and then split each of these two parts into two more substantially equal parts, thereby providing four substantially equal parts or controlled flow, thus allowing for a balanced volume flow to the die. Another feature of the crosshead extrusion system is the outside part or the body of the crosshead extruder which is designed to accept xe2x80x9cnon adjustablexe2x80x9d dies. Also the extruder is designed with dual controllable temperature zones in the front of the crosshead body to control the temperature of the material flowing through the body of the extruder.